add AsyncTaskLiveData class

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/ui/keyview/loader/AsyncTaskLiveData.java

@@ -0,0 +1,112 @@
+package org.sufficientlysecure.keychain.ui.keyview.loader;
+
+
+import android.arch.lifecycle.LiveData;
+import android.content.Context;
+import android.database.ContentObserver;
+import android.net.Uri;
+import android.os.AsyncTask;
+import android.os.Handler;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
+import android.support.v4.os.CancellationSignal;
+import android.support.v4.os.OperationCanceledException;
+
+public abstract class AsyncTaskLiveData<T> extends LiveData<T> {
+    @NonNull
+    private final Context context;
+    private Uri observedUri;
+
+    @NonNull
+    private final ForceLoadContentObserver observer;
+
+    @Nullable
+    private CancellationSignal cancellationSignal;
+
+    AsyncTaskLiveData(@NonNull Context context, @Nullable Uri observedUri) {
+        super();
+        this.context = context;
+        this.observedUri = observedUri;
+        this.observer = new ForceLoadContentObserver();
+    }
+
+    abstract T asyncLoadData();
+
+    private void loadDataInBackground() {
+        new AsyncTask<Void, Void, T>() {
+            @Override
+            protected T doInBackground(Void... params) {
+                try {
+                    synchronized (AsyncTaskLiveData.this) {
+                        cancellationSignal = new CancellationSignal();
+                    }
+                    try {
+                        return asyncLoadData();
+                    } finally {
+                        synchronized (AsyncTaskLiveData.this) {
+                            cancellationSignal = null;
+                        }
+                    }
+                } catch (OperationCanceledException e) {
+                    if (hasActiveObservers()) {
+                        throw e;
+                    }
+                    return null;
+                }
+            }
+
+            @Override
+            protected void onPostExecute(T value) {
+                setValue(value);
+            }
+        }.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
+    }
+
+    @Override
+    protected void onActive() {
+        T value = getValue();
+        if (value == null) {
+            loadDataInBackground();
+        }
+
+        if (observedUri != null) {
+            getContext().getContentResolver().registerContentObserver(observedUri, true, observer);
+        }
+    }
+
+    @Override
+    protected void onInactive() {
+        synchronized (AsyncTaskLiveData.this) {
+            if (cancellationSignal != null) {
+                cancellationSignal.cancel();
+            }
+        }
+
+        if (observedUri != null) {
+            getContext().getContentResolver().registerContentObserver(observedUri, true, observer);
+        }
+    }
+
+    @NonNull
+    public Context getContext() {
+        return context;
+    }
+
+    public final class ForceLoadContentObserver extends ContentObserver {
+
+        ForceLoadContentObserver() {
+            super(new Handler());
+        }
+
+        @Override
+        public boolean deliverSelfNotifications() {
+            return true;
+        }
+
+        @Override
+        public void onChange(boolean selfChange) {
+            loadDataInBackground();
+        }
+    }
+
+}